1. Field of the Invention
This invention relates to a method and apparatus for identifying a faulted phase or phases in a polyphase power line. More particularly, it relates to a method of identifying faults in a three-phase electric power system by vectorially subtracting the zero sequence current and prefault current from the postfault current to determine the fault type and phases involved.
2. Background Information
One of the functions of protective relaying systems used to isolate faults in electric power transmission lines is to identify the type of fault. Fault type identification includes determining if the fault is between phases or between one or more phases and ground and also determining the specific phase or phases involved in the fault. Proper faulted phase identification is required to avoid ground distance relay overreach on double line to ground faults which include ground fault resistance, for implementation of single-pole tripping for single line to ground faults, and for providing fault location information. The distance to the fault is computed from the fault impedance which is based on the voltages and currents of the faulted phases.
Current technology uses comparisons of different combinations of currents and voltages to determine fault type through mostly analog techniques. For example, previous phase selectors have selected the faulted phase based on phase comparison of zero sequence current and negative sequence current or voltage. These selectors identify as faulted the unfaulted phases during double line to ground faults. The selector utilizing zero and negative sequence currents may select two phases during a single line to ground fault if it is at a line terminal with substantial load but relatively small negative sequence current contribution to line faults.
One prior art phase selector based on phase comparison of zero sequence current, negative sequence voltage, and phase to phase voltage was intended to overcome the foregoing problems. However, it has been determined that, if ground fault resistance is involved, this phase selector may select the leading phase involved in a double line to ground fault and this is the situation where ground distance relay overreach may also occur.
The primary object of the present invention is to accurately identify the type of fault in a polyphase electric power line.
It is another important object of the invention to accurately identify the specific phase or phases involved in a fault.
It is also an object of the invention to achieve the previous objects regardless of fault resistance or load conditions.